Diisocyanates which are liquid at room temperature have numerous advantages over solid diisocyanates. Diisocyanates which are liquid at room temperature, such as toluene diisocyanate or hexamethylene diisocyanate, are, as a rule, physiologically harmful because of their high vapor pressure. For this reason, various attempts have been made to start with diisocyanates that are solid at room temperature and convert these into liquid form.
The most commercially important diisocyanates which are solid at room temperature are 4,4'-diphenylmethane diisocyanate and the 2,4'-isomer thereof, which melt at 39.degree. C. and 34.5.degree. C., respectively.
Numerous patents have issued relating to the liquification of diphenylmethane diisocyanate. See, for example, U.S. Pat. Nos. 3,152,162; 3,384,653; 3,394,165; 3,449,256: 3,640,966: 3,641,093; 3,674,828; 3,701,796; 3,883,571: 4,014,935: 4,055,548; 4,088,665; 4,031,026; 4,102,833; 4,115,429: 4,118,411: 4,154,752; 4,177,205; 4,229,347: 4,261,852; 4,321,333; 4,332,742; 4,490,300; 4,490,301; 4,490,302, 4,539,156; 4,539,157: and 4,539,158.
U.S. Pat. No. 3,644,457 (noted above) describes reacting 4,4'- and/or 2,4'-diphenylmethane diisocyanate with a branched aliphatic dihydroxy compound to produce a product which is liquid at room temperature. According to this reference, 1 mol of a diphenylmethane diisocyanate is reacted with from about 0.1 to about 0.3 mols of poly-1,2-propylene ether glycol to form a liquid product.
Liquid diphenylmethane diisocyanates have been produced by reacting diisocyanate having specified 2,4'-isomer contents with propylene and polypropylene glycols and with polyoxyethylene glycols (see, e.g., U.S. Pat. Nos. 4,118,411 and 4,115,429).
It has also been proposed to prepare liquid diphenylmethane diisocyanate compositions by reacting the diisocyanates with three separate alkylene glycols, each having at least three carbon atoms (see, e.g., U.S. Pat. No. 3,883,571), or by reacting the diisocyanate with at least three separate alkylene glycols, each having a least three carbon atoms, and wherein at least one of the glycols is dipropylene, tripropylene, or polypropylene glycol (see, e.g., U.S. Pat. No. 4,229,347).
Isocyanates based on diphenylmethane diisocyanate, tripropylene glycol and either a sucrose initiated polyether or a propoxylated glycerine are also known (see U.S. Pat. No. 4,442,235; note particularly Polyisocyanates I and J, columns 11 and 12).
It is an object of this invention to provide improved organic isocyanates which are liquid at 25.degree. C. A further object of this inention is to provide organic isocyanates which remain liquid even on prolonged storage. Still another object of this invention is to provide an improved process for preparing liquid organic isocyanates.